1. Field of the Invention
The present invention relates to a printing apparatus that is capable of adjusting dot misalignment in a main scanning direction with respect to dots created at different times, for example, dots printed in a forward pass and a backward pass of main scan, and to a method of adjusting such a misalignment of dots.
2. Description of the Related Art
Ink jet printers that eject ink from a print head to implement printing have widely been used as the output device of the computer. The ink jet printer generally moves a print head forward and backward relative to a printing medium as its main scan and causes multiple color inks to be ejected from the print head to create dots. Some of the ink jet printers create dots both in a forward pass and a backward pass of the main scan to enhance the recording speed (hereinafter this recording method is referred to as the bi-directional printing). In order to print an image of good quality, the dots created in the forward pass should be aligned with the dots created in the backward pass in the main scanning direction. When there is misalignment between the dots created in the forward pass and the dots created in the backward pass, a resulting image has harshness and poor image quality. Adjustment using a test pattern is typically carried out to compensate for such dot misalignment.
FIG. 24 shows a prior art test pattern, which is created by a print head HD having five nozzles. The left side of the drawing shows the position of the print head HD in a sub-scanning direction on the occasion of the forward pass of the main scan, and the right side of the drawing shows the position of the print head HD in the sub-scanning direction on the occasion of the backward pass of the main scan. In the central portion of the drawing, dots created in the forward pass are shown by the open circles, whereas dots created in the backward pass are shown by the closed circles. The process of printing the prior art test pattern first creates dots in the forward pass of the main scan, carries out sub-scan by a feeding amount L, which corresponds to an integral multiple N of a nozzle pitch k, and then creates dots in the backward pass of the main scan. The timing of ejecting ink is shifted by a unit step at each pixel in the backward pass, so as to vary the positions of the dots created in the backward pass relative to those in the forward pass. In the example of FIG. 24, the ink ejection timing in the backward pass is shifted by one through five steps respectively as indicated by Nos. 1 through 5. The user observes the printed results of the test pattern and selects the optimum dot positions, so as to adjust the ink ejection timing to cause no misalignment between the dots created in the forward and backward pass. In the illustrated example of FIG. 24, at the timing No. 3, the positions of the dots created in the backward pass are coincident with the positions of the dots created in the forward pass. Namely the timing No. 3 is optimum.
In order to attain the high image quality, the printers developed recently carry out printing at high resolutions using very small dots. Use of the very small dots, however, lowers the printing, speed. It is thus highly demanded to improve the image quality in the technique of bi-directional printing that enhances the printing speed. In the case of bi-directional recording, a slight deviation of the dot positions significantly affects the image quality of the resulting printed image. For example, when the print head has the tendency of deviating the dot positions leftward in the forward pass of the main scan from left to right, the dot positions are deviated rightward in the backward pass of the main scan. Namely the deviation is doubled in the case of bi-directional recording. Since the inappropriate adjustment of the dot positions in the forward pass and the backward pass of the main scan results in the extremely poor image quality in the case of bi-directional printing, the development of the technique has been highly desired to readily and accurately adjust the dot creation timing.
As the fruits of intensive experiments and discussions, the applicant of the present invention has found that the accurate adjustment of the misalignment of dots created in the forward pass with dots created in the backward pass significantly improves the image quality of the resulting printed image, which is equivalent to a significant increase in printing resolution. The increase in printing resolution using the very small dots undesirably raises the manufacturing cost of the printer. The arrangement of accurately matching the positions of the dots created in the forward pass with the positions of the dots created in the backward pass, however, readily improves the image quality without any increase in manufacturing cost. From these viewpoints, the technique of readily and accurately adjusting the dot misalignment in the case of bi-directional printing has been desired eagerly.
The test pattern shown in FIG. 24, however, does not attain the adjustment of the sufficient accuracy that satisfies these requirements. FIG. 25 is an enlarged view showing a test pattern actually printed. In the example of FIG. 25, dots are recorded by varying the ink ejection timing by one through fifteen steps respectively as indicated by Nos. 1 through 15. In each row of the printed test pattern, the upper portion includes only the dots created in the forward pass, and the lower portion includes only the dots created in the backward pass. An intermediate portion includes both the dots created in the forward pass and the dots created in the backward pass, which are overlapped with each other. The printed results of the test pattern shown in FIG. 25 show that Nos. 4 through 9 are in a preferable range of the dot creation timing. It is, however, very difficult to identify the optimum dot creation timing in this preferable range. Namely the prior art test pattern is not capable of adjusting the dot positions with a sufficiently high accuracy. In this example, the respective dots are printed at a relatively low resolution that allows the visual recognition. In the case of printing at a high resolution that causes each dot row to form a continuous line in the sub-scanning direction, it is almost impossible to specify the optimum dot creation timing.
Development of the technique for accurately adjusting the dot positions is highly desired especially in the case of bi-directional recording. The needs are, however, not restricted in the bi-directional recording, but also arise in a uni-directional printing, such as in the case of adjustment between multiple print heads of different colors and in the case of adjustment between dots of different ink quantities.
The object of the present invention is thus to provide a technique that readily and accurately adjusts the positions of dots created at different timings in a main scanning direction.
At least part of the above and the other related objects is attained by a print control apparatus that generates print control data and causes a printer unit to print a test pattern based on the generated print control data. The printer unit carries out main scan and sub-scan and creates dots with a plurality of dot-forming elements that have different positions in a sub-scanning direction. The test pattern is used to detect a misalignment of a plurality of dots created in each pixel by driving the dot-forming elements at different times.
The test pattern satisfies: a condition that the plurality of dots are created in a plurality of pixels having an identical position in a main scanning direction but different positions in the sub-scanning direction; and a condition that dots created at one time are respectively interposed between dots created at another time in at least part of the test pattern.
The printer unit receives the print control data generated by the print control apparatus and carries out printing of the test pattern specified above.
Here the plurality of different times may regard a diversity of cases, for example, the case of ejecting ink in one pixel by a plurality of different passes of the main scan and the case of ejecting ink in one pixel at preset time intervals in an identical pass of the main scan.
In the case where the printer unit is capable of creating dots in both a forward pass and a backward pass of the main scan, the test pattern is printed to detect a misalignment of dots created in the forward pass with dots created in the backward pass.
In the case where the printer unit has the plurality of dot-forming elements arranged in the sub-scanning direction and in the main scanning direction in a two-dimensional manner, the test pattern is printed to detect a misalignment of dots created by dot-forming elements having different positions in the main scanning direction.
When the printer unit uses a plurality of different color inks, for example, the arrangement in the two-dimensional manner may have the dot-forming elements that are aligned in one direction, either in the main scanning direction or in the sub-scanning direction, with regard to each color and arrayed in the other direction for the different colors. In another example, the dot-forming elements of an identical color may be arrayed not only in the sub-scanning direction but in the main scanning direction.
In the case where each of the plurality of dot-forming elements has a mechanism that is capable of ejecting an ink droplet at a varying jet speed to create a dot of a varying ink quantity, the test pattern is printed to detect a misalignment of dots formed by ink droplets ejected at different jet speeds.
One mechanism applicable to allow creation of dots having different ink quantities provides the dot-forming elements that are capable of consecutively ejecting an ink droplet having a higher jet speed and an ink droplet having a lower jet speed in each pixel, and selectively uses either one of these ink droplets. The consecutive ejection in each pixel is not essential here.
In the test pattern of the present invention, dots are created in. pixels having an identical position in the main scanning direction. Namely each dot row extends in the sub-scanning direction. The respective dots in each dot row have different positions in the sub-scanning direction, and are thus not completely overlapped with one another. There is a specific area in which the dots created at one time are respectively interposed between the dots created at another time. In the case where all these dots are created at appropriate positions, dots in each dot row are perfectly aligned in the sub-scanning direction. In the case where the dot positions are deviated from the appropriate positions, however, the resulting dot row is slightly jagged and has significant unevenness compared with the normal straight line. Using this test pattern to check for the presence of such unevenness enables the dot positions to be readily and accurately adjusted.
FIG. 1 shows an exemplified test pattern following the principle of the present invention. Like the prior art test pattern, dots are created both in the forward pass (dots shown by the open circles) and in the backward pass (dots shown by the closed circles) with five dot-forming elements arrayed in the sub-scanning direction. Five dot rows having numerals 1 through 5 allocated thereto are formed by shifting the drive timing of the dot-forming elements in the backward pass of the main scan in five different stages. Each dot row corresponds to the test pattern satisfying the conditions specified above, so that printing of only one dot row may be sufficient. As clearly seen from the printed results, dots are created at appropriate positions at the timing No. 3. The resulting dot row accordingly forms a straight line in the sub-scanning direction. The other timings Nos. 1, 2, 4, and 5, on the other hand, cause some misalignment and the resulting dot rows accordingly have significant unevenness. Adjustment of the drive timing of the dot-forming elements in the backward pass to the state of the timing No. 3 enables dots to be created at appropriate positions.
FIG. 2 is an enlarged view showing a test pattern actually printed according to the technique of the present invention. This test pattern is printed under the same conditions as those applied for the prior art test pattern shown in FIG. 25. Unevenness of the dot row is conspicuously recognized at the timings No. 1 and No. 15 having the misalignment of the dot positions. At the timings Nos. 5 through 7, on the other hand, there is little unevenness and the resulting dot rows are practically straight. The careful observation proves that the timing No. 6 gives a dot row having the best linearity. While the prior art test pattern can not specify the optimum timing among the timings Nos. 4 through 9 in FIG. 25, the test pattern of the present invention enables specification of the optimum timing.
Using the test pattern of the present invention facilitates detection of a misalignment of dots, mainly because of the following factors. In the prior art test pattern (see FIGS. 24 and 25), the overlapping degree of the dots created in the forward pass and the dots created in the backward pass increases with a decrease in misalignment of dots. The dot row having the less misalignment is thus visually recognized as the thinner line. In other words, the prior art test pattern specifies a misalignment using the thickness of the dot row as the index. The vision of the human is, however, rather insensitive to the difference in thickness, and can not detect a slight misalignment with a high accuracy. When there is little misalignment, dots are almost completely overlapped with each other. Blotting or stain may occur at the overlapped position. The blotting tends to thicken the dot row and makes it more difficult to detect the misalignment. The test pattern of the present invention, on the other hand, detects the misalignment, based on the degree of unevenness or linearity of the dot row. The vision of the human is generally very sensitive to the unevenness or linearity. The arrangement of the present invention accordingly enhances the accuracy of detecting the misalignment.
FIGS. 1 and 2 show the test pattern including the dots created in the forward pass of the main scan and the dots created in the backward pass. The test pattern of the present invention is applicable to detect a misalignment between a plurality of different dots created at different times. The technique of the present invention is not restricted to the dots created at two different times, but is applicable to the dots created at three or more different times, for example, dots created by three or more dot-forming elements having different positions in the main scanning direction. Although the dots are arranged at equal intervals in the sub-scanning direction in the example of FIGS. 1 and 2, the dots may be arranged at irregular intervals.
A variety of values may be set to the feeding amount of sub-scan in the process of printing the test pattern of the present invention.
For example, when the printer unit has the dot-forming elements at a pitch of k raster lines in the sub-scanning direction, where k is a natural number of not less than 2, the print control data causes the printer unit to carry out the sub-scan by a feeding amount. of s-k/m raster lines and print the test pattern, in order to detect a misalignment of dots created at m different times, where m is a natural number of not less than 2, s is equal to 1 or a natural number prime to the natural number m.
In this application, the printed test pattern includes dots arranged at equal intervals in the sub-scanning direction.
The test pattern of the present invention is used to detect the misalignment of dots, based on the unevenness or linearity of the dot row. It is accordingly desirable that the respective dots are individually recognizable. From this point of view, it is preferable that the test pattern includes dots arranged at such an interval that adjoining dots in the sub-scanning direction are not in contact with each other, although the test pattern is not restricted to this arrangement.
In the case where there is a sufficient interval between the adjoining dot-forming elements arrayed in the sub-scanning direction, all the dot-forming elements may be used to create dots in such a manner that the adjoining dots are not even partly overlapped with one another. In the case where the interval between the adjoining dot-forming elements is relatively narrow, there is a possibility of overlapping the adjoining dots. In this case, the test pattern may be printed by using only part of the plurality of dot-forming elements, which are not adjacent to one another. This arrangement gives a test pattern that ensures the accurate adjustment of the dot positions even when the dot-forming elements are arranged at an extremely high density for printing of the high resolution.
The drive timing of the printer unit may be regulated in a repeated manner while the test pattern is printed.
In accordance with one preferable application of the present invention, the print control apparatus further includes: a timing specification unit that causes a user to specify a desired drive timing of the dot-forming elements at each of the different times, based on the printed test pattern; and a modification instructing data generation unit that generates modification instructing data as data supplied to the printer unit to modify a preset drive timing of the dot-forming elements with the specified drive timing.
This arrangement enables the drive timing of the printer unit to be regulated to the appropriate value, based on the modification instructing data. It is also desirable that the printer unit has means for modifying the drive timing of the dot-forming elements based on the modification instructing data. When the drive timing of the dot-forming elements is directly controllable by the print control unit, the print control unit may execute a control procedure to modify the drive timing of the dot-forming elements with the timing specified by. the user.
The technique of the present invention is attained by a variety of applications other than the print control apparatus discussed above; for example, a printing system including a printer unit and the print control apparatus, a print controlling method, and a printing method, which follow the same primary concepts.
The technique of the present invention is also actualized by a recording medium, in which a program for causing the computer to carry out the functions discussed above is recorded in a computer readable manner. Typical examples of the recording medium include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed: thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. The present invention may also be directed to a program itself for attaining the functions discussed above, test pattern data, and a variety of equivalent signals.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.